Bad customer service

A few months ago l won voucher and l tried redeeming it in September but the voucher code did not work. l complained on Twitter, because that is were l won it but was told voucher was redeemed a few months earlier which l did not do as l only downloaded the app in September. l complained numerous times on twitter and Facebook with no response what so ever. Then about 2 weeks ago l was told to send an email but 2 weeks has gone by and still no response after sending my mail. l am very dissapointed with the service of a very big reputable company. l never expected to be completely ignored when this could have been sorted out a long time ago.